1898] REPRODUCTION OF THE ROTIFERA 49 
and similar organisms which swarm in such situations. It is thus 
very liable to be killed by drought, since it does not possess the 
power of withstanding drying exhibited to a marked degree by 
certain rotifers, But the rise of temperature preceding and causing 
the drying up of the water may not only act directly on the 
Hydatina as Maupas believes, determining the appearance of males 
and of resting-eggs, but it also brings into play the factor emphasised 
by Nussbaum by diminishing the supply of food, for the Luglenae 
encyst themselves and become aggregated into masses, in which 
state they cannot be ingested by the rotifers. Lauterborn considers 
that in Hydatina, and probably also in other species of similar 
habits, the power of ready response to changes in these two factors 
of its environment is a special adaptation ensuring the production 
of resting-eges when the colony is threatened with extermination by 
the drying up of its habitat. It has already been pointed out that 
in the Philodinidae, some of which live in a minimal quantity of 
water among damp moss or earth, the same end has been reached in 
another way, the adult animals becoming encysted in an envelope of 
hardened mucus, and in this state exhibiting an almost incredible 
power of resistance to prolonged drying. 
Lauterborn finds that the pelagic Rotifera may be divided into 
three categories :— 
(1) Perennial forms, which occur in greater or less abun- 
dance all the year round. 
(2) Summer forms, which are found only in the summer 
months. 
(3) Winter forms, including a few species whose occurrence 
is limited to the colder months of the year. 
The species belonging to the last two classes he finds to be 
strictly ‘monocyclic,’ in the sense in which Weismann has applied 
that term among the Cladocera; that is to say, the sexual period 
occurs only once a year, in autumn in the case of summer forms, in 
spring in the winter forms, when resting-eggs are produced to tide 
over the unfavourable seasons of winter and summer respectively. 
The ‘perennial’ species, on the other hand, are di- or poly- cyclic, the 
sexual period occurring at least twice in the year, in spring and in 
autumn, but without any interruption of the process of partheno- 
genetic reproduction, which goes on all the year round, so that the 
species never disappears from the gatherings. It is pointed out 
that those species which, in the pelagic fauna of lakes, show this 
perennial character, are for the most part forms known to occur also 
in smaller ponds and pools, while the monocyclic summer and winter 
forms are largely truly pelagic (limnetic) species inhabiting the open 
waters of larger ponds and lakes. These results show considerable 
agreement with those obtained by Weismann among the Cladocera. 
D 
